Summary The fetal and neonatal immune systems maintain a state of tolerance during pregnancy. However the moment that infants are born, their immature immune system must defend against the sudden onslaught of microbial pathogens present in the environment. Unfortunately for many, the transition from tolerance to protection is slow, providing windows of opportunity for specific infectious organisms. This susceptibility is best illustrated by group B Streptococcus (GBS), which is the most common pathogen in neonates but rarely leads to disease in healthy adults. Neonates are first exposed to GBS around delivery, as GBS frequently colonizes the birth canal. Despite efforts around screening and maternal treatment, GBS continues to be the major cause of neonatal pneumonia and early onset sepsis. To discover the mechanisms of GBS infection and immunity in the immature lung, we developed a novel mouse GBS pneumonia model. Adult mice clear GBS within 24 h, quickly resolve the initial lung inflammation, and universally survive GBS infection. However, neonatal mice fail to efficiently kill GBS, develop persistent lung inflammation and injury, and can die from GBS infection. Macrophages, the first line of defense in adult lungs, are immature in neonates making them vulnerable to inhaled and aspirated bacteria including GBS. Our preliminary studies show that immature neonatal lung macrophages fail to mount a normal immune response against GBS or kill phagocytosed bacteria. GBS avoids detection in the neonatal lung by expressing a capsule coated with sialic acid, mimicking ?self? antigens in the host. Neonatal lung macrophages express very low levels of sialoadhesin, which facilitates recognition of sialic acid in the GBS capsule and bacterial killing in adults. However neonatal lung macrophages do express Siglec-E, which suppresses inflammatory signaling upon sialic acid binding. This appears to represent a fetal tolerance mechanism, as the amniotic fluid is rich in the sialic acid modified Tamm-Horsfall Protein (THP). We hypothesize that THP and sialic acid maintain immune tolerance in the fetal lung through interactions with macrophage inhibitory Siglecs. For maturation of lung macrophages, THP-sialic acid-Siglec interactions must give way to the ability to detect and kill pathogens. GBS uses its sialic acid rich capsule to mimic the effects of THP, suppressing immune activation in newborn lungs. This proposal will use novel, state of the art approaches to further investigate the molecular mechanisms regulating both immune tolerance in fetal lungs and neonatal immunity against GBS. By bringing together outstanding expertise in the Principal Investigators? laboratories, state of the art approaches investigating development of lung immunity and mechanisms of host-microbe interactions will shed important new light on the ongoing battle between immunity and microbes around the time of birth. Collectively the aims in this proposal will identify unique molecular mechanisms that make newborns particularly susceptible to GBS pneumonia and explore new translational approaches for protecting newborns. The results will lead to development of new strategies for preventing and treating disease.